In the production of white cement the raw materials must have a high degree of purity and oil or gas (not coal) are used for firing in order to avoid impurities caused by ash. Moreover the firing is carried out in a reducing atmosphere in order to prevent the formation of the trivalent ferric ion which has a strong colouring power. In the production of white cement the following reduction reactions of colouring heavy metal oxides take place in a reducing kiln atmosphere: EQU Fe.sub.3 O.sub.4 +CO.fwdarw.3FeO+CO.sub.2 EQU Fe.sub.2 O.sub.3 +CO.fwdarw.2FeO+CO.sub.2 EQU MnO.sub.2 +CO.fwdarw.MnO+CO.sub.2 EQU 2CrO.sub.3 +2CO.fwdarw.Cr.sub.2 O.sub.3 +2CO.sub.2 +1/2O.sub.2
In the previously known methods for producing white cement, however, care must be taken to ensure that the CO content at the kiln outlet (i.e. in the region of the burner) is selected so that it is not too high, since otherwise the CO produced by the reducing atmosphere passes into the preheater where it is converted with difficulty, represents an additional heat loss and in certain circumstances represents a danger to the electrostatic filter.
In the known methods, therefore, in order to avoid these difficulties the reducing atmosphere is produced only in the outlet region of the rotary kiln, whilst in the remaining region of the rotary kiln the atmosphere is neutral or even contains an excess of oxygen.
The object of the invention is to provide a method of producing white cement in such a way that the reduction reactions referred to above take place particularly reliably and completely in the rotary kiln without the problems described above occurring in the preheater as a result of a CO content in the exhaust gases from the rotary kiln.